Figure 1. A schematic representation of conditions at
and beneath the surface of the tropical Pacific Ocean, extending from
the longitude of Australia on the left to the Americas on the right.
Contours are isotherms of sea-surface temperature, with warmer surface
temperature in darkest blue. The layer beneath the surface, labeled
Thermocline, separates the Sun-warmed water near the ocean surface
from the much colder waters of the deeper ocean below. Wide arrows along
the equator indicate the direction of prevailing surface winds. Dashed
lines depict the closed loop of vertical and horizontal circulation
of air above the ocean, driven by convection and marked by towering
tropical cumulus clouds. The upper diagram (a) is for "normal"
conditions. When an El Niño is in progress (diagram b), the westward-blowing
winds are reversed, warmer surface waters migrate eastward, and the
thermocline becomes shallower in the west and deeper in the east. Regions
of heavy tropical rain are also shifted eastward. (Based on a diagram
by M. McPhaden, NOAA/PMEL.)